Overload protective system



July 7, 1953 R. CRARY 2,644,563

OVERLOAD PROTECTIVE SYSTEM Filed March 10, 1950 LIMIT TORQUE DEGREES FROM HORIZONTAL g 6 o' 9'0 135' I 29 :50- air 3 7 assumes FROM uomzonm.

UNIT TORQUE E Fig.8 2 3 g C 6 +la6 INVENTOR. DEGREES FROM VERTICAL R. BY

ATTORNEYS Patented July 7, 1953 UNITED ns TA'TE S PATENT OFFICE l 2,644,563 l ,OYERLQAD iPROTECTlVE'SYSTEM 3 I Leonard R; Crary, Ifhilad elphia Pa. 1 Applieation aia 10, lestl jsrial No. 1 ,48,9s 6 1 efforts haveb'enmade to limit the powertransmission in'accordan'ce with thepowerrequirements of -the press as'fdescribed, for example, in United States BatentNoQIISMBQL ,S uch eiforts havenot generallybeen successful, however,'for a number of,reasonsamong whichi's the indefinite character ofthe, load imposed onapunch press. If the torque is. limitedto that necessary to c arry the ordinary load, objectionable work interruption Will -result from. frequent moderate I overloads that might not causedamage to the punch press. On the otherhand, if. torque limih tation; is at a sufficiently highlevel tocarryfnormal overloadsggbreakage of the punch press may ensue dueprimarily to the difijculty in fixing definitely the maximum load requirements.

, .Inaccordance .Withthe invention, and as an object thereof, the torque supplied to. a: punch pressor similar machine is lirnited, during its operating cycle to values somewhat less than the probablebreaking torque of; the machine rather than in accordance withits. probable load.

- This probable breaking torque of=a machine can be known-definitely since .it depends upon only the strength of the materials of which themachineis made and; not uponthe kind of ,work

. the machineis called upon to do. An, important advantage of this type ofprotection-isthe elimi nation I of unnecessary work interruptions due to unnecessary limiting of the torque suppliedto the punch press at some partsof its cycle.

A further object-of the, invention is to-protectpunch presses, and like machines, from breakage h nn e ar y:l mi ing he t rque-supplied thereto by momentum, thereby avoiding unnecessary work/interruption. 'lfhislpobjectis achieved in-one. respect by providing a torque Q limiting coupling, or; drive, at apoint in the power transmission systemibetween the crankshaft of the punch pressand any parts ,having' substantial inertia, It. is not. enough to disconnectthe punch press from itssourceof power in case of overload, if a, flywheel having .sub-

stantial momentum remains connected to the I punch presstotransmit excessive torque thereto.

Thus, in, accordance with, the ,invention;,,-the

. torque-limiting ,coupling. and; particularly that 6 Claims. (01 192-150) to protectivedevices for" portion thereof that is connected directly to the punchpressthat is to say, the portion of the ,couplingthat 'isf'notj connected directly to the 'powefsou'rce', is made small and compact in order to minimize its flywheel effect. Such, a-coupling ordinarily, will be interposed between the punch press and the usual clutch mechanism to thus minimize the flywheel effect of that portion'of the punch press which must stop abruptly in case of overload. A separate coupling-isdesirablefor the further reason that'the require- 15 punch-press clutch.

will be apparent from the following more detailed mentsfor a torque-limiting coupling di'fier materially from "the requirements of the usual Other objects and advantages of the invention description thereof with reference to the accompanying drawings, in which :1 V

Fig. 1 is a diagrammatic View of apparatus embodying the invention;

Fig. 2 is a fragmentary diagrammatic view taken substantially along the line 2-2 in Fig. 1;

' Fig, 3 is afragmentary vertical sectional detail view of a portion of the structure shown in Fig. 1; Fig. ,4 is 'a sectional view taken along lined-4 :i s- I I. 1 Fig. 5 is a diagram useful in explaining the invention illustrated in Figs. l'to .4 j

.. Fig. 6. is a diagram useful in explaining a modification of the invention;

.' Fig. 7is a fragmentary diagrammatic view .illustrating.'said modification of the structure shown in Fig. 2; and

; a Fig, 8 is a diagram useful in explaining the I invention.

Referring to Fig. 1, punch press It! comprises l a pdestal Hm, standards I01), and cap me. A dieQblock Ind is supported by pedestal Ina and, cooperating therewith in the usual way preferably guided by standards, 10b, is punch .iOe actuated by connecting rod 10f pivotally secured thereto 7 byfwrist pin lug. Connecting rod IE3 is actuated in awell-known way by crankshaft Iilh rotatably supported in any suitable bearings vI iii fixedly'se- .cured to standards lob.

beemployed, if desired. Motor I07 isadapted to be operated from any suitable power-supply such as electrical power line Him, Clutch ion interposed in the'mechanical power-transmission system between motor Illa and crankshaft lflh may spans-es 5 angular relation of crankshaft 1M and contact 120, but the sam "sequence would ensue if both contact I 2c and the resistance element were rotated together with respect to crankshaft IIlh.

Referring to Figs. 1,2, and 5, 'when'crank IIlp of crankshaft -I Uh is in its uppermost vertical position contact IQc-willengage conductor-portion 12d and the .full voltage of battery -F2afWill be effective to'produce current in coil Hi and rheostat I2e thereby causing drive unit II to transmit its maximum torque to shaft I Ie-and' then ce to crari-kshaft IBh. It is assumed for the present that there will be little-need for pro'tection ef punch press 10 with crank 10p in this position since punch l c is then fully raised and it must-desc'end somewhat before danger of breakageof-theipunch press occurs.

When crank Illp has rotated to its horizontal position contact I20 will still engage conductor portion I2d so that drive unit II will transmit its maximum torque as shownon a relative scale in Fig. 5 to 0 degrees from horizontal. In this position crank Illp may be said to have little leverage since little rotary motion of crankshaft IIJh will result in a relativel large displacement of punch Ille. Otherwise stated, a large torque applied to crankshaft Illh will resu'ltin relatively little force at punch 1-06.

For a clearer understandingof-the-reasons for torque limitations imposed in accordance with the invention, reference ismade to Fi-gr'B wherein the approximate relative torque required at crankshaft IDh to produce a .certain constant thrust at punch We 'is plotted. With crank in its uppermost orlowermost vertical position,

indicated at 0 or 180 degrees :from verticaL a -very small torque thereon theoretically would produce an infinite thrust at punch 106. Because the punch-press frame yields slightlythis greatleverage is never attained :in .practice'buta relatively small torque,.suoh as is indicated at A in Fi'gJB, will nevertheless produce a substantial force-Lat punch llle. As crank Hip rotates downward'to the horizontal, as indicated at 'plu :90 degrees from vertical in Fig. 8, its leverage decreases :so that a much larger torque, .such .as is indicated at B, is necessary to produce the 'sam'erforce at punch Hie. Thereafter, and of greater viimportance because this is the working stroke, a crank IOp rotates downward from the horizontal the torque thereon necessary to produce a given thrust at punch Ifie decreases to the value indicated at C.

Forrea'sons beyond the scope .of this :description, punch presses are actually-built to withstand only thrusts of the punch that'crankshaft 'IU'h is capable of exceeding duringthe lower part of its travel, below a certain criticalangle'of-crank I IJp. Seemingly it is not feasible to build punch-press frames and other parts that will withstand the great thrust which the :cranksh-afts are capable of applying to them when their leverage is great,

a? the torquere'quired to twist oif a shaft or-to 'strfp gears doe's-nottary. Onth'e-other hand-,=as-'crarik IiO'p is rotated downward from the critical angle the problem is to protect'the frame, -or'otl ier parts such as the crankshaft, from damage fromth'e thrustof -punch IIle and, therefore, the torque limit I of drive unit I I should gradually decrease because the torque necessary to produce a given thrust 'decreases to a minimum at the lowermost position-of c'rank Illp. 1

In agreement with the foregoing -discussion the maximum torque which drive unit l I-will transmit =as crank Illp rotates downward from its horizontal position should'be a constant determined by the torsional strength of the cranks'haftgortherotary members which drive it. constant torque limit is achieved, in'ac'cordanc'e with the invention, since "a constant current flows through coil IIi while contact 120 engages-conductor portion 12d, the current and hence the magnitude of the constant torque limit "being adjustable by rheostat I-Ze. When crank "I'Ilp reaches the aforesaid'critical angle, indicated at K in Figs. 2, 5, 6, and '7, at which danger'of torsional failure becomes-subordinate to the danger of failure of the punch-press frame, or other members, due to the thrust of punch Ille, the torque limitofdrive unit I I is gradually reduced by adding resistance portion I21 gradually to the circuit to reduce the current flowing through coil IIi.

Referring to Fig. 2, the'aforesaid'critical angle of crank Illp is'such that contact I20 leaves conductor portion 12d and engages resistance'portion I2f at the critical angle. As crank 10p continues to rotate to its lowermost .position where its leverage is a maximum resistance portion I 27 will all be inserted in the circuit'which includes battery IM and coil IIi so that the current through the coil, and hence the torque limit'of drive unit H, will be a minimum as required to protect the punchpress from excessive thrust of punch I lie. As crank 'lflp continues to rotate punch 'IIle will be raised through its upstroke where little danger of breakage ordinarily occurs and accordingly full torque is provided as contact 'I2c moves across resistance portion I2b to near the lowermost position of the cranks. 'When conductor .portion I20.

To avoid continuously short-circuiting resistance portions I2) and Nb, a small .gap 12h is left therebetween. However, it is not desired that the circuit including battery I 2a and coil .I Ii shall be interrupted ascontact I20 passesgap l2h, since such interruption would release drive unit II and the punch press would stop. 'To prevent such interruption contact [2c is made sufficiently wide to spangap I2h or, alternatively, it may be bifurcated, as shown schematically in the drawings, with two portions separated sufficiently .to span the gap 12h. A will'be clearly understood by those skilled in the art, the effect of such widening of contact I 20, or its. bifurcation, in addition to the desired effect of spannin gap 1 2h, willbe merely to short-circuit a s'hortsection of resistance portion IIiZf. If this effect isobjectionable it may be minimized by making .gap' I271. short and contact I 20 correspondingly narrow, or resistance portion [2) may be made correspondingly longer in the direction-of rotation of contact 'IZc. Resistance portions I21 and I2b may be tapered in a well-known way to bring about any desired pattern of torque variation.

Referring .to Fig. 5., the torque fromi) .to the aforesaid critical angle is limited 'to a constant.

of torque being that above which danger of fail ure, in torsion of some part of the drive system appears. From the critical angle of crank IDp to its lowermost position, 90 degrees from horizontal, the torque limit of drive unit II is gradually decreased by rotation of contact I20 thereby gradually interposing resistance portion I2 in the circuit of coil Hi to gradually decrease'the current flowing therethrough. Thereafter the torque limit of drive unit II will return to its aforesaid constant value when contact I20 engages conductor portion IZg.

Shortly after crank I 0p passes its lowermost position contact I2c momentarily engages both of resistance portions I2b and I2 so that parallel paths through these resistances are provided for current from battery I2a thereby momentarily doubling the current and doubling the limit torque of drive unit I I. Inasmuch as this increase in limit torque occurs at the beginning of the upstroke of punch me, it is improbable that any breakage of the punch press therefrom will occur. As will readily be understood by those skilled in the art, in some applications resistance portion IZb may be eliminated, conductor portions I2g being extended in place thereof, so that the limit torque of the machine will be increased abruptly to its maximum value at the beginning of the upstroke of punch ice. Ordinarily, however, resistance portion IZb may have about the same resistance as resistance portion I2f to produce the variation in limit torque shown in Fig. 5. In the operation of certain machines, such for example as a double acting punch press wherein the work piece is successively engaged by a plurality of punch elements, it is desirable to decrease the limit torque of the drive unit at more than one place in the operating cycle of the machine' For convenience suppose that, in a double-acting toggle press the toggle acts slightly'be fore the crank reaches its horizontal position and that thereafter the crank-actuated punch operates. Such double-acting toggle presses are well known and need not be described further herein. In Fig. 6 the desired pattern of limit torque is illustrated, the decrease in limit torque at 90 degrees from horizontal correspondingto that shown in Fig. 5 and serving to protect the punch press from the great thrust on punch We made possible by the great leverage of crank IIlp near the bottom of its stroke The second decrease in limit torque shown in Fig. 6 at about 330 degrees from horizontal, or otherwise stated slightly above the horizontal position of crank I020, serves to protect the punch press from damage due to the thrust of a toggle-actuated punch element, not shown.

To provide the limit-torque pattern shown in Fig. 6 rhecstat I3, shown in. Fig. '7, is substituted for rheostat I2 shown in Fig. 2. Referring to Fig. '7, when crank lop has started downward and the toggle action is beginning, contact I20 engages resistance portion I37 which is short circuited by conductor I3k. As contact I2c moves toward'the middle of resistance portion 539', resistance gradually is introduced into the circuit including battery I2a and coil IIi progressively to reduce the limit torque of drive unit II. As contact I2c passes the middle of resistance portion I37, corresponding to about 330 de rees from horizontal in Fig. 6, it approaches conductor I370 gradually reducing to zero the aforesaid resistance in the circuit including battery I2a and coil I Ii thereby increasing to its maximum, constant value the limit torque of drive unit I I. As crank Hip passes its horizontal position the operation of the system is exactly like that previously described with reference to rhecstat I2 until con-.

tact I2c again engages resistance portion I37. It will readily be understood by those skilled in the art that other resistance portions similar to resistance portion I37" short circuited by conductor I37; may be interposed in conductor portions. 1311 or 13g to decrease the limit torque of drive unit II at other places in the operating cycle of the machine, if desired.

While preferred embodiments of the invention have been described it will be understood that modifications thereof may be made within the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. In a system for protecting from damage due to overload a power-driven machine having the characteristic of being susceptible to such damageonly at driving torques above a limit which changes through recurring cycles of operation of the machine, said torque limit being substantially constant during a certain portion of said cycles, the combinationwhich comp-rises an electric drive unit for transmitting to said machine torque less than an adjustable torque limit determined by the magnitude of an electric current supplied to said unit, said unit being incapable of transmitting to said machine torque greater than said adjustable limit, and means for recurrently adjusting said current concurrently with said cycles, said means being adapted and arranged to adjust said current to maintain said adjustable torque limit substantially at said driving torque limit throughout said portion of said cycles, said means comprising a rhecstat including relatively movable contact and resistance members, one of said members being driven relative to the other in synchronism with said machine, said resistance member including a constant current conducting portion for maintaining constant said adjustable torque limit during said portion of said cycles and a progressively decreasing resistance portion for progressively decreasing said adjustable torque limit during other portions of said cycles.

2. In a system for protecting from damage due to overload a power-driven machine having the characteristic of being susceptible to such damage only at driving torques above a limit which changes through recurring cycles of operation of the machine, the combination which comprises an electric drive unit for transmitting to said machine torque less than an adjustable torque limit determined by the magnitude of an electric current supplied to said unit, said unit being incapable of transmitting tosaid machine torque greater than said adjustable limit, and means for recurrently adjusting said current concurrently with said cycles, said means being adapted and arranged to adjust said current to maintain said adjustable torque limit substantially at said driving torque limit throughout certain portions of said cycles, said means comprising a rhecstat including relatively movable contact and resistance members, one of said members bein driven relative, to the other in synchronism with said machine, said drive unit comprising a pair of engageable clutch elements having oblique substantially frictionless clutch faces for transmission therebetween of force directed only substantially tromagnet for opposing by an adjustable force said separation of said elements to adjust the magnitude of said axial force component required to disengage said elements thereby adjustably limiting the torque transmittable by said drive, said electromagnet being adapted and arranged to apply said opposing force in accordance with the magnitude of said electric current.

3. In a system for protecting from damage due to overload a power driven machine having the characteristic of being susceptible to such damage only at driving torques above a torque limit which changes during the working stroke of a cycle of operation of the machine, and wherein the torque limit is constant and then progressively decreases during the working stroke of the cycle, the combination which comprises an electric drive unit for transmitting to said machine torque less than a torque limit determined by the magnitude of an electric current supplied to said unit, said unit being incapable of transmitting to said machine torque greater than said torque limit, and means for maintaining the current to said unit constant during a portion of the working stroke to a predetermined point and then progressively decreasing the current for the remaining portion of the working stroke, said means comprising a rheostat including relatively movable contact and resistance members, one of said members being driven relative to the other in synchronism with said machine, said resistance member including a contact portion for maintaining constant the current during the portion of the working stroke to said predetermined point and a progressively increasing resistance portion for decreasing the current during the remaining portion of the Working stroke.

4. In a system of the type described in claim 3 wherein the resistance member of said rheostat includes a progressively decreasing resistance portion connected to said first resistance portion and said conducting portion for progressively increasing the current to said unit during a portion of the non-working stroke of said cycle.

5. In a system for protecting from damage due to overload a power driven machine having the characteristic of being susceptible to such damage only at driving torques above a torque limit which changes during the working stroke of a cycle of operation of the machine, and wherein the torque limit is constant and then progressively decreases during the working stroke of the cycle, the combination which comprises an electric drive unit for transmitting to said machine torque less than a torque limit determined by the magnitude of an electric current supplied to said unit, said unit being incapable of transmitting to said machine torque greater than said torque limit, and means for maintaining the current to said unit constant, during a portion of the working stroke to a predetermined point and then progressively decreasing the current for the remaining portion of the working stroke, said means comprising a current flow controlling device including an element operated in synchronism with cycle of operation of the machine, and wherein the torque limit is constant and then progressively decreases during'the working stroke of the cycle, the combination which comprises an electric drive unit for transmitting to said machine,

torque less than a torque limit determined by the magnitude of an electric current sup-plied to said unit, said unit being incapable of transmitting to said machine torque greater than said torque limit, and means for maintaining the current to said unit constant during a portion of the working stroke to a predeterminedpoint and then progressively decreasing the current for the remaining portion of the working stroke, said means comprising a current flow controlling device including a current controlling element and a current transfer element, one of said elements being driven relative to the other in synchronism with said machine, said current controlling element cooperating with said current transfer element to deliver current to said electrical drive unit at a constant rate during that portion of the working stroke to said predetermined point and to deliver current at a progressively decreasing rate during theremainingportion of said working stroke.

LEONARD IR. CRARY.

References Cited in the file of this patent UNITED STATES PATENTS Number France Oct. 20, 1930 

